The present invention relates to an improved method for producing a layer of a protective lacquer, which improves the blocking properties and stabilizes the surface of a semiconductor device on the edge surface of a semiconductor device in the region where the pn-junction or junctions of the device exit. More particularly the present invention relates to an improved method for providing such a protective lacquer layer wherein a plurality of semiconductor wafers is placed, in a mutual spacing which is appropriate for their further processing, so that one of the contact or major surfaces of each wafer is on a planar support made of plastic material, the plastic material is then heated until it becomes soft and then permitted to cool so as to hold the wafers in place, thereafter a protective lacquer is introduced between the semiconductor wafers which lacquer encloses the pn-junction(s) coming to the surface, and finally the lacquer foil which forms after hardening of the protective lacquer together with the semiconductor wafers embedded therein is removed from the support.
In the production of semiconductor devices, particularly semiconductor rectifier devices with a high blocking voltage carrying capability, it is particularly important to cover the semiconductor surface in the region where the pn-junction(s) exit with a substance which protects the surface against environmental influences and which improves and stabilizes the electrical properties of the device, particularly the blocking properties.
A method for producing a protective coating on semiconductor surfaces is known according to which semiconductor wafers are arranged in large numbers and at mutual spacings with one of their surfaces on a planar support of plastic material which has been heated beyond its softening point and, after cooling of the support, a protective lacquer which improves the blocking properties and stabilizes the semiconductor surface is introduced in liquid form between the semiconductor wafers. After hardening, this protective lacquer forms a continuous foil which encloses the semiconductor wafers in the region where the pn-junction exits and is then removed from the support together with the semiconductor wafers. The semiconductor wafers intended for this treatment each preferably comprises a disc-shaped semiconductor body having a metallic contact plate fastened to each of its two major surfaces.
In this known process, the semiconductor wafers may sink into the softened plastic support beyond the pn-junction to be protected. This has the result that after removal from the support the respective surface region of the semiconductor wafers is unprotected and may be subjected to damaging influences from the environment. Moreover, large-area semiconductor wafers cannot be fixed in the desired manner in the support due to the greatly varying coefficients of thermal expansion of the wafer contact material and the material of the support so that after cooling of the support the wafers often lie only loosely on top of the support and the protective lacquer, when it is being applied, may creep under the edge zones of the contact areas where electrical contact is to be made. This leads to undesirable wetting of the contact surface and requires additional work. Finally, when the semiconductor wafers are removed from the support according to the known process, there exists the danger that due to insufficient adhesion to the wafer periphery the protective lacquer foil will tear away from the surface it is to protect so that expensive additional treatment of the semiconductor wafers becomes necessary.